Most living organisms are aerobic; that is, they require oxygen to complete the total breakdown of glucose. As many as 36 ATP are produced through aerobic metabolism of one glucose molecule. Without oxygen, cells do not synthesize enough ATP to maintain a multicellular organism. Most people think that we need oxygen to breathe, but actually we need oxygen to recycle the spent electrons and hydrogen ions (H+) produced as by-products of aerobic respiration. With these by-products, oxygen combines and forms “metabolic” water.